1. Field of the Invention
The present invention relates to a method for manufacturing an optical fiber preform used to make optical glass fibers. In particular it relates to a method for increasing the deposition rate of a process by suitably redistributing a flow of glass particles before it impacts onto a target preform.
2. Background Art
Glass fibers for optical communication are made from high purity, silica-based glass fibers drawn from glass preforms, which preforms are produced according to various glass deposition techniques.
Some of these deposition techniques, including vapor axial deposition (VAD) and outside vapor deposition (OVD), are based on flame combustion wherein reactants (i.e. silica precursors, such as SiCl4, optionally together with dopants materials, such as GeCl4, for suitably modifying the refractive index of the glass) are fed together with combusting gases through a deposition burner which directs a high temperature flow of forming fine glass particles onto a rotating growing target preform.
According to the VAD deposition technique, the growth of the preform takes place in an axial direction. Thus, the deposition burner(s) is typically maintained in a substantially fixed position, while the rotating preform is slowly moved upwardly (or downwardly) with respect to the burner, in order to cause the axial growth of the preform. Alternatively, the rotating preform can be maintained in a substantially fixed position, while the deposition burner is slowly moved downwardly (or upwardly) with respect to the preform.
Differently from the VAD technique, in the OVD technique the growth of the preform takes place in a radial direction. In this case, a rotating target (e.g. a quartz glass rod) is generally positioned in a fixed horizontal or vertical position and the deposition burner is repeatedly passed along the surface of the growing preform for causing the radial growth of the same.
Independently from the applied deposition technique, a porous glass preform is thus fabricated, which is then consolidated to form a solid glass preform apt for being subsequently drawn into an optical fiber.
Typically, an optical preform comprises a central portion (core) and an outer portion (cladding), the core and the cladding differing in their respective chemical composition and having thus different refractive indexes. As in the optical fibers, the cladding portion forms the majority of the preform. The preform is typically manufactured by producing and consolidating a first preform comprising the core and a first portion of the cladding. An overcladding layer is then deposited onto said first preform, thus obtaining a porous preform, which is then consolidated into the final preform.
In general, conventional burners for manufacturing optical preforms are made up of a plurality of co-axial pipes through which the glass precursor materials (i.e. silica precursors, such as SiCl4, optionally together with dopants materials, such as GeCl4), the combusting gases (e.g. oxygen and hydrogen or methane) and, optionally, some inert gas (e.g. argon or helium) are fed. Typically, the glass precursor material is fed through the central pipe of the burner, while other gases are fed through the annular openings formed by the concentrically disposed pipes.
Examples of such conventional burners are disclosed, for instance, in U.S. Pat. Nos. 4,345,928, 4,465,708, 4,474,593, 4,661,140, and 4,810,189.
“Multi-flame” burners, generating a plurality of independent flames disposed concentrically one to each other, are also disclosed. For instance, U.S. Pat. No. 4,801,322 discloses a multi-flame burner wherein the inner flame, including a glass precursor material, is positioned rearwardly of the outer flame. As mentioned in said patent, the outer flame allows to increase the flame length with consequent size increase of the synthesised glass particles.
U.S. Pat. No. 4,826,520 discloses a modified multi-flame burner for producing doped optical preforms wherein a central pipe, through which a doping reactant (GeCl4) is fed, is spaced forwardly with respect to the other pipes forming the inner flame, in order to reduce the staying time of the doping material inside the flame.
With the increasing demand for optical fibers, there is now the need of manufacturing optical preforms of larger dimensions and in a more effective and fast manner.
As observed by the Applicant, while the burners for depositing the core and the inner cladding of the preform are generally of reduced dimensions, burners used for depositing the overcladding, in particular for large dimensions preforms, shall be relatively larger, in order to allow the generation of the higher flow rates necessary for increasing the amount of deposited material, maintaining at the same time the velocity of the gases relatively low.
The Applicant has now observed that, particularly for cladding or overcladding deposition (in the specific according to the VAD deposition technique) and especially when manufacturing large optical preforms, the deposition rate of the process can be increased by suitably redistributing the flow of fine glass particles impacting onto the target preform. This can be done by suitably modifying the geometry of the deposition burner. In particular, it has been observed that the shape of said flow can be advantageously modified in its terminal portion before impacting onto the target preform, by increasing the dimension of said flow in a direction substantially perpendicular to the longitudinal axis of said target preform.